The principal aims of this proposal are to follow the morphological, physiological and biochemical differentiation of individual neurons in the relatively simple, segmented central nervous system of the medicinal leech. This animal offers the advantage that it can be bred in the laboratory and develops rapidly over a period of about three weeks. In addition, much is already known about the properties of cells in the adult. The experiments will be made using electrophysiological procedures, intracellular dye injections, and microchemical techniques developed for the adult nervous system and shown to be practicable in embryos. One key problem will be to determine if the ontogeny of individual neurons of known function is characterized by fixed developmental sequences. For example, every neuron assumes its own stereotyped morphology, establishes precise synaptic connections, synthesizes, stores, and releases a particular chemical transmitter, and is selectively sensitive to certain other transmitters and hormones. Is there a specific series of steps by which each cell acqires its characteristic phenotype, how rigid is the sequence for a particular neuron, and how does the sequence vary among different nerve cells? Which features are expressed independently of functional contacts, and how do cellular interactions such as the arrival of synaptic inputs, interaction with target tissues, competition with other cells, and patterns of electrical activity influence differentiation? In other experiments normal development will be disrupted, either pharmacologically, by eliminating target tissues, or by destroying individal cells. Analysis of the effects of such manipulations at the cellular level will indicate more precisely how interactions among cells affect their differentiation. Our hope is that information obtained in this simplified preparation will provide clues for understanding general mechanisms controlling development.